During injection molding, molten thermoplastic material flows into a mold cavity of a mold via one or more gates. The pressure within the mold cavity is an important vector, as insufficient pressure may result in improperly formed parts while excessive pressure may result in damage to the mold. The pressure at the melt flow front provides information relevant to the injection molding process overall by, for example, enabling calculation of how fast a mold cavity is filling and how long cooling of the molded part within the cavity is likely to take. Some injection molding processes aim to have the melt flow front follow a particular pressure pattern over time in order to optimize the injection molding process. For example, some injection molding processes maintain a pressure balance between the air pressure in the cavity, which changes based on the mold cavity geometry as the molten thermoplastic material moves into the cavity of the mold, and the pressure at the melt flow front, in order to produce a final product that is fully relieved of internal stresses that would otherwise undesirably lead to shrink, sink and warpage. For such injection molding systems, determining the real-time pressure at the melt flow front is important in order to determine whether the desired pressure/time curve is properly being followed and, if deviations from the pressure/time curve are identified, to make adjustments to correct the pressure of the melt flow front.
One particular motivation to monitor flow front position and/or control internal pressure at certain times during the short duration of filling of a mold cavity is to account for flow filling challenges. The term “flow filling challenge” is defined as a region of a part of a mold that forms a feature of a part to be molded which is particularly susceptible to any one or more of a number of problems that complicate the molding of the part or render the molded part more likely to suffer from one or more defects or reduced mechanical properties, such as short-fills, warp, sinks, brittleness, flash, voids, non-fills, weakness (e.g., low tensile, torsional, and/or hoop strength), high stress concentrations, low modulus, reduced resistance to chemical exposure, premature fatigue, non-uniform shrinkage, and discontinuities in color, surface texture, opacity, translucency, or transparency. Non-exhaustive examples of flow filling challenges are: Locations in a mold used to form ribs, bosses, or corners, as well as obstacles in a mold (such as core pins), and transitions (such as a change in thickness of a part to be molded, which may be a sudden stepped change in thickness or a gradual change in thickness, such as a tapered region). These can involve a transition from a relatively thick region to a relatively thin region, and then back to a relatively thick region, and may involve one or more changes in thickness. The portion of a mold cavity used to form a living hinge, which is typically an integral, relatively thin region of a molded part that permits one portion of the part, such as a flip-top of a cap, to rotate with respect to the rest of the part, poses a flow filling challenge. As the term flow filling challenge is used herein, it is contemplated that the region of the part affected by a particular challenge may be at a particular position of a mold cavity, along a region of a mold cavity, or downstream of a particular position or region of a mold cavity, and as such, a flow filling challenge need not be limited to a particular location of a change in shape of a mold cavity, but may extend beyond, i.e. downstream of, such a location.
Ideally, sensors for measuring the pressure within a mold cavity and at a melt flow front would be indirect, easy to install, and inexpensive. Direct sensors, such as sensors placed within a mold cavity, leave undesirable marks on part surfaces. For example, while demand for injection molded parts with high gloss finishes has been increasing, direct sensors positioned in the mold cavity have a tendency to mar the high gloss finish of the parts. As a result, indirect sensors that are not located in the mold cavity are preferable. Some current indirect sensors include parting line sensors, ejector or static pin sensors, and ultrasonic sensors. Unfortunately, these indirect sensors cannot always be placed in optimal locations, sometimes require that a mold apparatus undergo a period of downtime in order to be machined so that the sensor can be mounted, and can be expensive. Strain gauge sensors have been used in the past in conjunction with molding apparatuses having ejector sleeves or long core pins, but not all injection molding apparatuses are configured to include an ejector sleeve or long core pin.
More recently, external sensors, such as strain gauges, have been placed on a mold surface in order to measure how a condition, such as strain, changes over the course of a standard injection molding process. In a typical injection molding apparatus, a mold cavity is formed between two mold sides, which are held together under pressure by a press or clamping unit. Thus, along the parting line of the mold, a closing force is exerted by the press or clamping unit. When molten thermoplastic material is injected into the mold cavity, the molten thermoplastic material exerts an opening force along the parting line of the mold. Ideally, the opening force exerted by the molten thermoplastic material is less than the closing force exerted by the clamping unit. If the opening force is greater than the closing force, the mold sides are forced apart and flashing, or leakage of the molten thermoplastic material, occurs. A strain gauge sensor placed on the exterior of the mold surface adjacent to a parting line of a mold is able to sense the surface strain changes on the mold surface that occur over time as a result of the closing and opening forces. In response to surface strain changes, the strain gauge sensor emits an electrical signal, typically in the range of −10 to 10 Volts. The signal emitted by the strain gauge sensor is received and used by a controller to approximate one or more conditions within the mold, such as the pressure within the mold cavity or the location of the melt flow front. In certain molds in which the ratio of the length of the flow channel to the thickness of the molded part is great, i.e. molds having a high length-to-thickness (L/t) ratio, the pressure at the melt flow front may be approximated based on the signals emitted by the strain gauge sensor(s). These approximations may be useful for adjusting the injection molding process. For example, the amount of pressure within the mold cavity may be approximated and compared to a maximum permissible mold cavity pressure in an effort to ensure that the mold cavity is not damaged by excessive mold cavity pressure.
However, two key challenges make it difficult to approximate a condition within a mold cavity using an external sensor, such as a strain gauge, placed on a mold surface. First, not all areas of a mold surface experience a measurable condition, such as strain, in a way that accurately, reliably, and/or quantifiably corresponds with a condition within the mold, such as the pressure within the mold cavity or the location of the melt flow front, and therefore only some areas of a mold surface can be used to approximate one or more conditions within the mold, such as the pressure within the mold cavity or the location of the melt flow front. Injection molds come in a variety of shapes and sizes. Identifying areas of a mold surface that can be used to approximate a condition within the mold using an external strain gauge sensor has required testing a number of different areas on the mold surface, which can be time consuming, or making a blind guess that may turn out to be wrong. Second, even in areas on the surface mold that do experience a condition, such as strain, in a way that corresponds with a condition within the mold, external sensors often pick up some amount of “noise” generated during the molding process that does not correspond with a condition within the mold cavity. The noise measurements picked up by the external sensor must be distinguished from the meaningful measurements in order to accurately approximate one or more conditions within the mold. In some cases, the ratio of noise measurements to meaningful measurements is so high that conditions within the mold cannot be accurately approximated.